Electrically operated devices are so ubiquitous that most people benefit from a wide variety of electrical devices during each day of their lives. Indeed, electrically operated devices have become so common in the home that most modern homes have about forty such devices other than lights. These devices include; alarm clocks, lights, heaters, shavers, radio and television, toasters, oven ranges, toothbrushes, air conditioners, typewriters, calculators, and a wide variety of appliances and tools for the household, kitchens and shop, as well as devices used in connection with leisure hours such as swimming pool filters and lights, grills, rotisseries, fire starters, and many other devices. The wide spread distribution and use of electrical devices greatly increases the possibility of shock and many accidents and deaths resulting from misuse of such devices and/or from defective and/or worn devices have occured. It has been common practice to provide fusing and/or circuit breakers to provide some protection. However, while fuses and circuit breakers have, for the most part, done an admirable job of protecting equipment, they have failed to provide adequate protection for personnel, inasmuch as people can be seriously and fatally injured by a current which is much smaller than the current which is required to flow to trip a circuit breaker or blow a fuse. Accordingly, various techniques have been employed to try to protect personnel and reduce the probability of electrical shock. One technique, which was introduced years ago and which is now in common usage, involves the use of a grounding wire. The grounding wire (not to be confused with the power conducting grounded wire) is a third wire which is employed to ground the conducting housing of the associated electrical device. The use of a grounding conductor has helped to save many people from serious or fatal electrical shock. However, the use of the grounding conductor has not entirely eliminated the occurance of serious electrical shock. Accordingly, electrical engineering expertise has been used to devise a means to protect personnel by disconnecting power whenever there is an indication tht current exceeding a few milliamperes is flowing elsewhere than in the power conducting leads.
In the normal power distribution circuit the power conducting leads comprise a live conductor and a grounded conductor. If things are functioning properly, the currents in the two conductors are equal. Various techniques have been devised for detecting any difference of current in the live and grounded conductor and for opening the circuit in response to any difference of current. The structure described herein is directed to the class of devices which is intended to be sensitive to any difference of current in a live and grounded conductor and for disconnecting power from the load when there is a difference of current exceeding a predetermined value. Tests have indicated that a difference of more than approximately 5 milliamperes between the current in the live and neutral (grounded) conductors constitutes a possible hazard. Accordingly, a design criteria is to affect disconnection of power to the protected load in response to an unbalanced current of approximately 5 milliamperes.
A differential transformer is a convenient device for detecting an unbalance of current in two wires. In a differential tranformer, each of the wires to be compared couples the transformer with an equal number of turns (usually one) and are wound in sense such that the magnetomotive force generated by each winding exactly cancels the magnetomotive force of the other winding, provided both windings have equal current. A secondary winding is included on the differential transformer and a potential is inducted in this secondary winding whenever there is an unbalanced current in the other windings. The potential induced in the secondary winding has been used to trip circuit breakers and thereby disconnect a load in response to an unbalanced current. This technique requires the use of a sensitive circuit breaker which can respond to the relatively small signal available at the secondary of the differential transformer. Unfortunately, such systems do not respond with sufficient consistency to provide a satisfactory and economical operating technique.
An alternate technique has been proposed using a differential transformer and the induction of a high frequency tickler voltage in the neutral conductor. Another proposed technique involved the use of a means for increasing the difference of current between the live and neutral conductors in response to a small difference therebetween to thereby increase the power available at the secondary of the differential transformer.
The various devices of the prior art suffered from a variety of faults including being too sensitive, not sensitive enough, too costly and too bulky.